This is something of a non-story, I think (by which I mean not that the subject matter of the piece isn't important, or that the event that prompted Mr Hendy's documentary isn't tragic, but that it doesn't lead anywhere). Mr Hendy is asking the same question that anybody who has had a loved one taken from them, particularly in violent, untimely circmstances would ask... why? The only difference being that Mr Hendy was able to make a documentary, and get it airtime on national TV.
Scale of mental health homicides 'is being played down'
Obviously, whether the number is 50 or 100 mentally ill murderers, per year, that's quite a lot. However, I see no comparative for "sane" murderers... What proportion of the mentally ill vs the sane commits murders, each year? To be honest, I'd be surprised if the mentally ill were any more violent than the sane... In fact, I'd suggest that the mentally ill just aren't quite as good at hiding their violence. Or perhaps they deliberately make no attempt to hide it...
Anyway, as I wrote: something of a non-story.
If I were to extrapolate from the little I know of this story, I would say that Mr Hendy would find it easier to deal with the death of his father, if he knew that he had been killed in a botched robbery, by a "sane" person. From this, I can only assume that Mr Hendy believes that the mentally ill person who killed his father did so for no reason at all - (s)he just suddenly decided, in complete isolation to everything else that was going on in his/her mind, that murder must be done, and Mr Hendy's father got in the way. That scenario is not possible...
Mr Hendy snr may well have been unfortunate to be in the wrong place, at the wrong time, but there was nothing illogical about what the murderer did, given the way that (s)he understood the world. But as to how and why I know that... Well, that's my secret.